This invention relates to the field of automatic valves for controlling gas passage through conduits, and in particular to the field of automatic flap valves used in the feed ducts of two-stroke internal combustion engines.
Conventional flap valves are known to consist of an elastic flap adhering tightly to the edge of a hole or port until a gas emerges from the interior of said hole at a pressure which creates a force able to deflect said elastic flap, thus also overcoming any pressure which previously tended to keep the flap adhering to the edge of the hole in order to close it.
In this type of conventional valve an essential role is played by the material and shape of the flap, as the valve operation depends totally on the flap bending resistance.
The pressure difference which can exist between the one and the other side of the flap has influence only during an initial theoretical stage, or only when this pressure is of kinetic order. In this respect, from a static viewpoint as soon as the flap lifts it becomes surrounded by the gas on all sides, and is therefore in contact with only one pressure.
Again, in this conventional type of valve the gas flow undergoes considerable trajectory deviation in passing through it, so that it suffers pressure drops which can be very damaging. In the case of internal combustion engines this pressure drop proportionally reduces the volumetric efficiency. These pressure drops are not only due to the friction deriving from change in trajectory but also due to a multiplicity of irregularities to which the fluid flow is subjected within the duct because of the intrinsic material structures typical of this conventional type of valve.
A further drawback of conventional valves arises because of the need for their flaps to have considerable elasticity and fatigue resistance. This results in high cost deriving from the materials used and the precise machining required to give the flaps the necessary reliability, and also means that these latter must be of considerable weight to satisfy the strength requirements.
In this respect, their weight results in an inertia which prevents the flaps from operating in high frequency fields such as those towards which modern two-stroke engines are tending, this field constituting a sector of useful application for said valves.